Voltage efficiency of alkaline water electrolysis by using a mixed metal oxide cathode catalyst

ABSTRACT

A mixed metal oxide electrode for alkaline water electrolysis. The mixed metal oxide comprises Ir, Ru, and a non-noble metal selected from periods 4, 5 and 6 of the periodic table such as W, Ce, Ni, V and Co.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims the benefit of and priority to U.S. Provisional Application Ser. No. 62/051,982 filed Sep. 18, 2014, entitled “Improving the Voltage Efficiency of Alkaline Water Electrolysis by Using a Mixed Metal Oxide Cathode Catalyst,” which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to a mixed metal oxide cathode catalyst for use in alkaline water electrolysis.

BACKGROUND OF THE INVENTION

The electrolyzer is a device that generates hydrogen and oxygen from water through the application of electricity and consists of a series of porous graphite plates through which water flows while low voltage direct current is applied. Electrolyzers split the water into hydrogen and oxygen gases by the passage of electricity, normally by breaking down compounds into elements or simpler products. An electrolyzer has to fulfil requirements such as high efficiency, low cost, large range of operation etc. Physically a practical electrolyzer stack will consist of several cells linked in series. Electrolysis cells are characterized by their electrolyte type. There are two types of low temperature electrolysis: alkaline electrolysis and proton exchange membrane (PEM) electrolysis.

Generally, the efficiencies of proton exchange membranes are higher than alkaline electrolyzers due to the presence of a membrane. The half reaction taking place on the anode side of a PEM electrolyzer is commonly referred to as the Oxygen Evolution Reaction (OER). Here the liquid water reactant is supplied to catalyst where the supplied water is oxidized to oxygen, protons and electrons. The half reaction taking place on the cathode side of a PEM electrolyzer is commonly referred to as the Hydrogen Evolution Reaction (HER). Here the supplied electrons and the protons that have conducted through the membrane are combined to create gaseous hydrogen. The reactions at the electrodes are as follows:

-   -   Anode reaction: H₂O→½O₂+2H⁺+2e⁻     -   Cathode reaction: 2H⁺+2e⁻→H₂     -   Overall cell reaction: H₂O→H₂+½O₂

In contrast alkaline water electrolysis is a type of electrolyzer that is characterized by having two electrodes operating in a liquid alkaline electrolyte without a membrane. Although alkaline water electrolysis is a more mature technology than proton exchange membranes electrolysis it is plagued by the need of high electrical consumption costs. Common types of electrolyte used include KOH and NaOH. The reactions at the electrodes are as follows:

-   -   Anode reaction: 2HO⁻→½O₂+H₂O)+2e⁻     -   Cathode reaction: 2H₂O+2e⁻→H₂+2HO⁻     -   Overall cell reaction: H₂O→H₂+½O₂

As shown above the reactions at the electrodes are completely different between alkaline water electrolysis and proton exchange membrane electrolysis. To elaborate on this fact the hydroxide ions produced in the cathode reaction of the alkaline water electrolysis cannot be produced at the cathode reaction of the proton exchange membrane. Therefore electrode materials described in patent applications geared towards proton exchange membranes, such as 2003/0057088, cannot be used as alkaline water electrolysis electrodes.

There exists a need to improve upon alkaline electrolysis process for hydrogen production by improving the electrode catalyst activity and lowering the electrical consumption cost.

BRIEF SUMMARY OF THE DISCLOSURE

A mixed metal oxide electrode for alkaline water electrolysis. The mixed metal oxide comprises Ir, Ru, and a non-noble metal selected from periods 4, 5 and 6 such as W, Ce, Ni, V and Co.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a scanning electron micrograph of IrO₂ catalysts synthesized using the aqueous hydrolysis method.

FIG. 2 depicts a scanning electron micrograph and corresponding energy dispersive x-ray spectrogram for Ir_(0.33)Ru_(0.33)Co_(0.33)O₂ catalysts.

FIG. 3 depicts the voltage efficiencies of cobalt-based cathode catalysts.

FIG. 4 depicts the voltage efficiencies of different mixed metal oxide cathode catalysts.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

The term “electrolysis” describes the process of passing electricity through water to liberate its elemental parents, hydrogen and oxygen gas. A basic water electrolysis unit consists of two electrodes (an anode and a cathode), a power supply, and an aqueous electrolyte solution. A direct current (DC) is applied by the power supply across the two electrodes, causing electrons to flow from the negative terminal of the DC source to the cathode at which the electrons are consumed by hydrogen ions (cations) to from hydrogen. Maintaining charge balance, hydroxide ions transfer through the electrolyte solution to the anode, where they give up electrons to form oxygen. These electrons are then captured at the positive terminal of the DC source, completing the circuit. Overall, water is being reduced to dihydrogen at the cathode and oxidized to dioxygen at the anode.

The present embodiment is directed towards a mixed metal oxide electrode for alkaline water electrolysis. The mixed metal oxide comprises Ir, Ru, and a non-noble metal selected from periods 4, 5 and 6 such as W, Ce, Ni, V and Co.

Although it is feasible that the mixed metal oxide electrode could be used for both the cathode and the anode of the alkaline water electrolysis it is anticipated that the cathode would use the mixed metal oxide electrode while the anode would use typical metals associated with alkaline water electrolysis. Typical metals for alkaline water electrolysis include sole elements and mixtures of: titanium, nickel or zirconium.

Catalyst Synthesis

Salts of the required metals were weighed corresponding to the desired ratios in the final product. The salts were dissolved in 25 ml of ultrapure water and stirred at room temperature. 0.2 g of NaOH was added to the salt, resulting in an increase of the pH beyond 12. This solution was stirred and heated at a temperature of 80° C. for 1 hour resulting in the formation of hydroxides or oxides of the corresponding metals. 1 M HNO₃ was added to the resulting solution in order to reduce the pH below 8 and oxidize the remaining metal compounds. The precipitate was collected upon centrifugation at 4000 r.p.m. for 5 minutes in two cycles, and ethanol/water was added to the collected solids to prepare a catalyst ink with a solids concentration of 10 mg/ml.

Table 1 below describes a list of representative chemicals that can be used for the catalyst synthesis.

TABLE 1 Chemical Name Chemical Formula Purity (%) Dihydrogen Hexachloroiridate H₂IrCl•4H₂O 99.90 Ruthenium Chloride RuCl₃•xH₂O 99.99 Cobalt Nitrate Hexahydrate Co(NO₃)₂•6H₂O 99.00 Sodium Hydroxide NaOH 99.10 Nickel Nitrate Hexahydrate Ni(NO₃)₂•6H₂0 99.00 Cerium Ammonium Nitrate (NH₄)₂Ce(NO₃)6 >98.50 Ammonium Vanadium Oxide NH₄VO₃ 99.93 Sodium Tungsten Oxide Dihydrate Na₂WO₄•2H₂O 99.00-101.00 Nitric Acid HNO₃ 96.20 Ethanol C₂H₅OH 95.00 Oxalic Acid Dihydrate C₂O₄H₂•2H₂0 99.98

Table 2 below describes the atomic percentages of Ir, Ru and Co intended in the catalysts and the corresponding weights of the precursors taken to synthesize 0.25 mmols of final product.

TABLE 2 Sam- Ir Ru Co H₂IrCl•4H₂O RuCl₃ Co(NO₃)₂•6H₂O ple (%) (%) (%) (mg) (mg) (mg) 1 50 50 0 60 26 0 2 44 44 12 52 23 9 3 37.5 37.5 25 45 19 18 4 34 34 33 40 17 24 5 25 25 50 30 13 36 6 12.5 12.5 75 15 6 55 7 0 0 100 0 0 73

FIG. 1 depicts a scanning electron micrograph of IrO₂ catalysts synthesized using the aqueous hydrolysis method. The sizes of the nanoparticles are in the range of 10 to 50 nm.

FIG. 2 depicts a scanning electron micrograph and corresponding energy dispersive x-ray spectrogram for Ir_(0.33)Ru_(0.33)Co_(0.33)O₂ catalysts. The spectrogram confirms the presence of Ir, Ru and Co in the solids in the desired ratios.

Electrode Fabrication

Titanium foils were all used as substrates for catalyst deposition. All the electrode materials were cut to approximately 1 cm×2 cm coupons. The coupons were etched in 10% oxalic acid at a temperature of 80° C. for 30 minutes. The electrodes were then cleaned with DI water, 100% ethanol and DI water again and blown dry using compressed air. The catalyst ink was ultrasonicated in a bath for 5 minutes to disperse the solids. The dispersed ink was then deposited on the etched Titanium electrode drop-wise as a suspension (100-200 microliter) followed by covering the substrates by placing a watch-glass upside-down on them to ensure the slow-drying of the solvent. All the electrodes mentioned here had a catalyst loading of 1 mg/cm².

The dried films were then placed in a ceramic boat and heated in a tube furnace in an inert argon atmosphere at a temperature of 500° C. for 1 hour. Each electrode was then sealed with a ‘5-minute Epoxy’ by covering 80% of the back side of the substrate and half the area on the front side, leaving approximately 1 cm² of exposed catalyst area on the front. The epoxy was then left to set at room temperature for 15 minutes.

Electrode Testing

The deposited mixed metal oxide-based electrode was used as the cathode in the electrolysis of water to produce hydrogen and oxygen. The electrolysis of water was carried out in a 50 mL beaker by electrically connecting the anode (SS foil) to the cathode via the potentiostat. The anode was connected to the positive terminal and the cathode to the negative terminal of the power source, and an inter-electrode spacing of 0.5 cm was used. The anode, cathode and a 30% w/w KOH electrolyte were placed in a 50 mL beaker having means for heating and agitating (magnetic stirring) the solution. The temperature of the beaker was maintained at 80° C. by heating on a heater-plate. Constant currents of 100 mA, 200 mA, 300 mA, 400 mA, 500 mA and 600 mA were used for measuring the voltage efficiencies. The corresponding cell voltages were recorded by the computer.

FIG. 3 depicts the voltage efficiencies for the different cathode (hydrogen generation electrode) materials using 316 stainless steel as the anode material. Tests run in a beaker with 30% KOH, 5 mm electrode spacing at 80° C. The green curve shows the efficiency of commercially available Ni—Fe cathodes. In-house developed IrRuCo oxides demonstrated a higher efficiency than the commercially developed electrodes as shown in FIG. 3, with Ir_(0.25)Ru_(0.25)Co_(0.5)O₂ showing the best efficiency of 86%.

FIG. 4 depicts the voltage efficiencies of catalysts based on other non-noble metals such as W, Ce, Ni and V, all of which demonstrated higher efficiencies than typical commercial cathodes.

Table 3 below depicts the voltage efficiency of various cathode catalysts with a SS anode Tests were performed in a beaker in 30% KOH at 80° C. and 300 mA/cm² with 5 mm electrode spacing.

Cathode Efficiency Baseline 70.0 Ir_(0.33)Ru_(0.22)V_(0.33)O₂ 77.0 Ir_(0.38)Ru_(0.38)V_(0.25)O₂ 74.4 Commercial NiFe electrode 77.5 Ir_(0.33)Ru_(0.22)W_(0.33)O₂ 79.1 Ir_(0.33)Ru_(0.33)Ce_(0.33)O₂ 79.3 Ir_(0.33)Ru_(0.33)Co_(0.33)O₂ 79.6 Ir_(0.5)Ru_(0.5)O₂ 81.5 Ir_(0.44)Ru_(0.44)Ni_(0.12)O₂ 83.1 Ir_(0.5)Co_(0.5)O₂ 83.7 Ir_(0.25)Ru_(0.25)Co_(0.5)O₂ 85.6

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents. 

1. An electrode for alkaline water electrolysis comprising: a mixed metal oxide comprising of: Ir, Ru, and a non-noble metal selected from the group consisting of: elements selected from periods 4, 5 and 6 of the periodic table.
 2. The electrode of claim 1, wherein the non-noble metal is selected from the group consisting of W, Ce, Ni, V and Co.
 3. The electrode of claim 1, wherein the electrode is a cathode electrode.
 4. The electrode of claim 1, wherein Ir and Ru are at equimolar ratios of each other.
 5. The electrode of claim 1, wherein hydroxide ions are produced by the electrode during the alkaline water electrolysis.
 6. The electrode of claim 1, wherein the non-noble metal is Co.
 7. The electrode of claim 1, wherein the mixed metal oxide consists essentially of: Ir, Ru, and a non-noble metal selected from the group consisting essentially of: elements selected from periods 4, 5 and 6 of the periodic table.
 8. The electrode of claim 1, wherein the molar ratio of Ir:Ru:non-noble metal is 0.25:0.25:0.50.
 9. An cathode electrode for generating hydroxide ions for an alkaline water electrolysis comprising: a mixed metal oxide consisting essentially of Ir, Ru and Co in a molar ratio of 0.25:0.25:0.50. 